I think a lot of people talk about how the desktop is customizable, but what I would like to see happen is a conformity in laptop parts so that a large range of upgrade optioms are available in the laptop form factor. Like, if I wanted to replace my HP laptop's keyboard with one the same size, but maybe backlit, that would be awesome. Or if I could remove the video card and replace it with a better version.
I think if these types of things come to be common, yes indeed I think the desktop will start to see a decline. I can take my laptop and hook an external keyboard or mouse to it, but to tell you the truth in no way do I miss my desktop machine.